Conventionally, there is known a four-wheel drive vehicle in which a driving force of a drive source is transmitted to front wheels at all times while transmitting a required amount of driving force according to the driving conditions of the vehicle to rear wheels via a driving force transmission unit (for example, refer to Patent Document 1).
In a four-wheel drive vehicle described in Patent Document 1, torque transmitted via a propeller shaft is split laterally by a rear gear mechanism to be distributed to rear wheels via a clutch provided at an output side of the gear mechanism. This clutch is pressed in an axial direction by virtue of a thrust produced by a cam mechanism having a first and second cam members which can rotate relatively to thereby transmit the torque to the rear wheels by means of frictional engagement force of a plurality of inner plates and a plurality of outer plates which are disposed coaxially and alternatively. In addition, this cam mechanism is disposed between an outer rotational member which is connected to an output shaft of the gear mechanism and an inner rotational member which is connected to the rear wheels. The first cam member receives torque of the outer rotational member via an electromagnetic clutch of which engagement force can be controlled by a current amount of an electromagnetic coil, and the second cam member is connected to the inner rotational member so as to prohibit relative rotation therebetween. Then, when torque is transmitted to the outer rotational member with the electromagnetic coil energized, part of the torque is transformed into axial thrust by the cam mechanism so as to press the clutch.